1. Field of the Invention
This invention relates to a unit for vaporizing liquid materials, more particularly, to a unit for vaporizing liquid materials which is preferably used for supplying liquid materials for film formation to a semiconductor manufacturing unit.
2. Discussion of the Relevant Art
A process of forming a thin metal film on a base is indispensable for manufacturing semiconductor devices. Conventional process of forming a thin metal film is the PVD process (the Physical Vapor Deposition process) such as a vacuum deposition process or a sputtering process. However, the CVD process (the Chemical Vapor Deposition process or the Chemical Vapor Phase Growth process), especially the MOCVD process (the Metalorganic Chemical Vapor deposition process) has recently been well utilized. The MOCVD process is to vaporize a volatile metal organic compound to be formed into a thin film or a liquid of the metal dissolved into a solvent (hereinafter the above mentioned compound or the liquid will comprehensively be called a liquid material), to convey the vaporized liquid material to a surface of a base in a state of vapor and to separate out a thin film of the decomposition product by means of a reaction such as a pyrolysis. The MOCVD process is considered superior to the conventional processes in several points such as a film quality, a velocity of film formation and a step coverage.
A conventional unit for vaporizing liquid materials comprises a plurality of containers each of which contains a material dissolved into a solvent, a plurality of liquid conveying lines to gather the liquid materials from each containers to one place and a vaporizer to vaporize the liquid materials conveyed through the liquid conveying lines. With the arrangement of the above-mentioned conventional unit, the mixing rate of the liquid materials can be changed if a flow control valve is arranged in a liquid conveying line. However, in order to dilute or concentrate the mixed liquid material each containers should be dismantled one by one to change the mixing rate of the solvent and the liquid material. This makes it complicated to operate the unit. In addition, this kind of the liquid material is liquid under normal temperatures and pressures, and hydrolysis may take place if the liquid material is touched by atmospheric moisture when the container is opened. This makes it extremely difficult to handle the liquid material.
Upon changing the mixing rate of a solvent and a liquid material it sometimes happens that a predetermined mixing rate is not obtained accurately by just adjusting a flow of the liquid material proportionally to the mixing rate since each of the liquid materials practically has a different characteristics such as compressibility and viscosity.
It is known that conventional vaporizers have such an structure that a liquid material contained in a reservoir is pumped up by a constant amount with a pump, the liquid material is absorbed into a mesh-shaped heating element having a flash matrix structure, the heating element is heated so as to vaporize the liquid material, and the vaporized material is supplied together with carrier gas to a CVD unit for film formation.
In accordance with the vaporizers having the above-mentioned structure, however, if an amount of the liquid material pumped from the pump is increased so as to vaporize a large quantity of the liquid material, a heating element of the vaporizer is cooled by the pumped liquid material itself, resulting in deteriorating thermal efficiency. As a result, the amount of the liquid material to be vaporized can not be increased.
It is an object of the present invention to provide a unit for vaporizing liquid materials which can easily and precisely adjust not only a mixing rate but also a concentration of the mixed liquid and which makes it possible to effectively vaporize a large quantity of liquid material.